This invention generally relates to metal oxide semiconductor field effect transistors (MOSFET), and in particular the present invention relates to an improvement applicable to the MOSFET structure in all of its currently known embodiments. Further this invention relates to the methods used to fabricate the improved MOSFET. The MOSFET which is the subject of this invention and referred to hereinafter as an enhanced mobility MOSFET or EMMOSFET, exhibits more rapid switching characteristics, higher band width, and lower heat generation.
The operation of the metal-oxide-field effect transistor requires the application of a voltage to a gate electrode situated parallel to the surface of a substrate and electrically isolated from it by an insulating oxide layer. If a p-type or alternatively an n-type substrate is used then electrons or alternatively holes are the minority carriers in the bulk of the silicon wafer. However, the application of a positive or alternatively a negative voltage causes the electrons or alternatively holes to be swept to the semiconductor surface directly under the gate electrode thus making them majority carriers in a thin (less than about 100 .ANG.) layer. The thin layer of carriers (carriers refer to electrons or holes) is arranged to bridge the area between two electrical contacts called the "source" and "drain" and provide electrical conductivity in between the source and the drain.
The speed of a MOSFET is a measure of how fast the change of the gate voltage causes a change in the electrical current between the source and drain. Attempts have been made to increase the speed of MOSFETS since high speed is an important attribute for MOSFETS especially where they are used in digital circuits. Efforts to improve the speed of MOSFETS were mainly directed toward making the MOSFET smaller.
In the scaling down of the device size, all of the geometrical dimensions are reduced by a scaling factor (1/K) but at the same time the threshold and punch through voltage must be maintained which can be achieved by reducing the power supply voltage by K and increasing doping concentration by K. However, a reduction of the power supply voltage has been found to be unacceptable since the reduction of voltage would necessitate extensive hardware changes. Hence, the industry standard of 5 V for the power supply has been retained, and will probably be retained in the foreseeable future. The further scaling of the devices to smaller sizes will not yield an optimum set of device parameters when the devices are operated using the 5 V standard gate voltage. Nevertheless the reduction of the gate length from 3.5 micrometers (.mu.m) to 2.0 .mu.m has reduced access time from 35 nanoseconds(ns) to 15 ns. In other words the reduction in gate length has increased the device speed by a factor of more than two. Because of the need to further increase device speed, it would likely be difficult for industry to maintain the 5 V single-voltage standard for power supply voltage in IC manufacture if the efforts to increase speed were to continue along the lines of device size reduction.
There is a fundamental difficulty associated with gaining device speed by size reduction without an accompanied voltage reduction. When the source-drain voltage is kept fixed and the size reduced, the electric field between the source and the drain increases causing a velocity saturation with a consequential decrease in electron mobility and an increase in power or energy dissipation. Hence it is important to increase the speed of devices to the maximum possible extent without reverting to the scaling down of the device size so as to cause velocity saturation. Such an effort has the obvious advantage of enabling industry to stay with the 5 V power supply standard and consequently not have to change hardware external to the IC package., Also extensive retooling with new and expensive equipment for photolithographic processes suitable for smaller sizes will not be required. This invention provides a means to increase the device speed without further device size reduction thus allowing the continued use of the 5 V power supply and the currently accepted hardware and packaging. The printed circuit (PC) boards, connectors, etc. need not be changed. Further by keeping the device size and power supply voltage unchanged, it will not be necessary for industry to retool with new mask pattern generators and mask aligners, and other such production facilities. This invention, rather than decreasing the path length from source to drain, provides a means to increase the speed of travel of the carriers from source to drain.